1. Field of the Invention
The present invention relates to an image reading apparatus.
2. Discussion of the Background
An electrographic image forming apparatus includes an image reading apparatus using a stationary-document-reading method (a platen-set method). The stationary-document-reading method reads an image of a printed document placed on a platen glass of a document table by exposure and scanning of an optical system. Japanese Patent No. 3660180 and Japanese Unexamined Patent Application Publication No. 2010-220055 disclose what is called a shading correction to correct for density variation (light amount variation) in the main scanning direction caused by characteristics of an optical system. The shading correction is a correction that includes: locating a white reference plate in an upstream, which is outside of a document placing area, of the sub scanning direction; reading a white color on the surface of the white reference plate with an optical system, so as to obtain shading correction data; and correcting image data such that the image data has uniform brightness of a printed document on a pixel to pixel basis, based on the shading correction data.